1. Field of the Invention
The present invention relates to an ink composition for ink jet recording, and an ink set comprising the same.
2. Background Art
Ink jet recording is a printing method wherein droplets of an ink composition are ejected and deposited on recording media, such as paper, to print, for example, letters or figures. The feature of the ink jet recording method is that images having a combination of high resolution with high quality can be printed at a high speed by means of a relatively inexpensive apparatus.
Examples of ink jet recording methods which have been put to practical use include: a method wherein an electric signal is converted to a mechanical signal using an electrostrictive element to intermittently eject an ink reservoired in a nozzle head section, thereby recording letters or symbols on the surface of a recording medium; and a method wherein an ink, reservoired in a nozzle head section, in its portion very close to the ejection portion is rapidly heated to create a bubble and the ink is intermittently ejected by volume expansion created by the bubble to record letters or symbols on the surface of a recording medium.
Various properties required of ink compositions used in the above ink jet recording are such that the drying property of the print is good, no significant feathering is created in printed images, uniform printing can be realized on the surface of conventional recording media, and, in the case of multi-color printing, mixing between adjacent colors does not occur. Further, what is important to ink compositions is to ensure satisfactory ejection stability of ink at actual service temperatures of ink jet recording apparatuses.
To ensure these properties, various studies have hitherto been made on additives to ink compositions.
For example, they have proposed the use of an additive for lowering the surface tension of the ink composition to increase the penetration speed of the ink composition into paper and thus to reduce feathering or bleeding. More specifically, the use of diethylene glycol monobutyl ether (U.S. Pat. No. 5,156,675) or Surfynol 465 (manufactured by Air Products and Chemicals Inc., U.S.A.) (U.S. Pat. No. 5,183,502) as an acetylene glycol surfactant or the combined use of diethylene glycol monobutyl ether and Surfynol 465 (U.S. Pat. No. 5,196,056) has been proposed. Further, an ink composition has been proposed which contains, as a wetting agent, polyglycerin (Japanese Patent Laid-Open No. 152170/1991), polyglycerin with an ethyleneoxy group added thereto (Japanese Patent Laid-Open No. 328644/1997), or glycerin with an ethyleneoxy group added thereto (Japanese Patent Laid-Open No. 18465/1992).
For these conventional ink compositions, however, there is still room for improvement. Specifically in the case of printing on plain paper, for example, mere addition of an adduct of ethyleneoxy groups with glycerin to ink compositions sometimes causes prolongation of drying time due to high surface tension. Therefore, when printing is continuously carried out on a plurality of sheets, there is fear that satisfactory drying time cannot be ensured. Thus, immediately after printing, prints cannot be put on top of one another. This is disadvantageous in high-speed printing. Further, this ink composition cannot in some cases produce high-quality prints.
An ink jet recording method using an electrostrictive element is advantageous in that printing can be carried out without thermal damage to the ink composition. Further, a recording head using an electrostrictive element has a great advantage such that the head can eject ink droplets several hundred millions of times and thus can be permanently used. In ink using a relatively large amount of solid matter, such as colorants, when the ink is not ejected for a long period of time, the ink dries on the front face of the nozzle in the recording head and thus is thickened, often leading to print disorder. By contrast, according to the head using an electrostrictive element, this problem can be overcome by delicately moving the ink at the front face of the nozzle on such a level that does not eject the ink from the nozzle, thereby stirring the ink. Therefore, the ink jet recording method using the electrostrictive element is advantageous in recording using an ink composition containing a high concentration of a colorant, such as a pigment.
Japanese Patent Laid-Open No. 157698/1995 discloses examples of ink compositions to which a 1,2-alkylene glycol has been added to reduce bleeding of the dye. This publication, however, does not disclose pigment-based ink compositions. In general, as compared with dyes, pigments are advantageously superior in waterfastness, lightfastness, and weatherfastness, and, at the same time, is less likely to cause color mixing. Further, so far as the present inventors know, mere use of the 1,2-alkylene glycol in ink compositions is likely to cause clogging in a head using an electrostrictive element.
Japanese Patent Laid-Open No. 18465/1992 discloses an ink composition containing some of the compounds represented by formula (I) which will be described later. This ink composition is used in an ink jet recording method wherein ink is ejected from an orifice of the recording head through the action of thermal energy to perform recording. In this case, the compound represented by formula (I) is used for improving the ejection stability of the ink in this recording method. This publication, however, does not disclose the combined use of the compound represented by formula (I) and glycerin.